1. Technical Field
The present disclosure generally relates to the field of photocontrols and, more particularly, low-power photocontrols used with luminaires.
2. Description of the Related Art
A photocontrol is a device that switches or controls electrical loads based on ambient light levels. As an example, a photocontrol can be used as a switch that provides electrical power to a luminaire only when detected light levels are below a desired level. Photocontrols used for such luminaires may include photosensors that are electrically and operably coupled to switching devices rated for use at relatively high line voltages (e.g., 90 VAC to 600 VAC) and at relatively high currents (e.g., amperes and higher). For example, a photocontrol for a luminaire may include a photosensor that controls an electro-mechanical relay coupled between a source of electrical power and a control device (e.g., a magnetic or electronic transformer) within the luminaire. The electro-mechanical relay may be configured to be in an electrically continuous state unless a signal from the photosensor is present to supply power to the luminaire. If the photosensor is illuminated with a sufficient amount of light, the photosensor outputs the signal that causes the electro-mechanical relay to switch to an electrically discontinuous state such that no power is supplied to the luminaire.
Conventional photocontrols used with luminaires suffer from a number of drawbacks. For example, such photocontrols may include small power sources that use “capacitive drop” technology to power a circuit of discrete transistors, integrated circuit operational amplifiers, or comparators. Conventional photocontrols using such technology can consume considerable amounts of power when the luminaire is ON and when the luminaire is OFF.
Additionally, a typical electro-mechanical relay used with a photocontrol for a luminaire has a relatively short life span. For example, electro-mechanical relays of conventional photocontrols used with luminaries may be rated to have only 5000 contactor closures with standard loads. Arching caused by high capacitive in-rush currents of electronically ballasted luminaires and inductive “kick back” of magnetically ballasted luminaires can corrode the contactors of the electro-mechanical relays. Additionally, the contactors may include silver or other metal alloys upon which oxides and sulfides may form during normal operation. At line voltage and current, such oxides and sulfides may present a negligible resistance to the passage of current through the contactors. However, at relatively low voltages (e.g., 2 V to 24 V) and relatively low currents (e.g., μA) such as those used for digital logic level signaling, the impedance presented by contaminants including oxide and sulfide accumulations can hinder or even prevent the transmission of current through the contactors. Thus, conventional photocontrols for luminaires can be unsuitable for use in applications where the switching of relatively low voltage and relatively low current signals is required, for example, with luminaires that include solid-state light source drivers, for example, light emitting diode (LED) drivers that receive control signals for dimming LED arrays.
In response to the increasing emphasis placed on energy efficiency, many luminaires are being retrofitted with more energy efficient light sources. For example, conventional light sources (e.g., incandescent lights) are being replaced with solid-state light sources (e.g., LED arrays). Circuitry that regulates electrical power supplied to such solid-state light source (e.g., LED drivers) may draw relatively high inrush currents when the light sources are switched on. The inrush currents of electrically ballasted light sources may cause more damage to the contactors of electro-mechanical relays than is caused by the kickback currents of magnetically ballasted light sources. Accordingly, when conventional photocontrols having electro-mechanical relays are used with luminaires having solid-state light sources, the electro-mechanical relays may fail or cease to function reliably well before their rated number of contactor closures.
There is therefore a need for photocontrols that consume very small amounts of power. Additionally, there is a need for photocontrols that can be used reliably over long periods of time with luminaires having solid-state light sources.